1. Field of the Invention
The invenetion relates to a method of growing silicon monocrystal by Czochralski method, and more particularly to a method of growing silicon monocrystal by applying cusp field to molten silicon.
2. Description of the Related Art
A conventional method of growing silicon monocrystal by Czochralski method has a problem that when a silicon monocrystal wafer used for a substrate on which a ultra-micro, and ultra-highly integrated device is formed is to be grown, if a wafer has an ununiform oxygen concentration profile, the wafer would be heavily curved in heat treatment.
Another problem is that if a wafer has fluctuation in a quite small concentration of oxygen, there would be generated unevenness in oxygen precipitation during device fabrication steps, which in turn causes reduction in a fabrication yield of devices.
The above-mentioned fluctuation in a small concentration of oxygen forms a pattern in crystal. This pattern is called a growth slit, and can be observed, for instance, by An X-ray topograph.
The growth slit is caused by small fluctuation in temperature at an interface between solid and liquid phases in growing crystal. The growth slit has a shape reflecting a shape of an interface between solid and liquid phases, in a direction in which molten silicon is drawn up, and has a shape of concentric circles in a plane of crystal.
An oxygen concentration varies in dependence on a profile of the growth slit. Oxygen contained in a silicon monocrystal wafer is unpreferably mixed into silicon crystal due to melting of a quartz crucible which contains molten silicon therein while silicon monocrystal is grown.
Accordingly, it is quite important in silicon monocrystal growth to ensure uniformity in oxygen concentration profile in crystal, and to suppress fluctuation in a small concentration of oxygen, namely, to prevent generation of growth slits.
To this end, Japanese Unexamined Patent Publications Nos. 62-83348 and 1-282185 have suggested a method of growing silicon monocrystal by Czochralski method. In the suggested method, cusp field is applied to molten silicon, and a revolution per minute of a crucible and molten silicon contained in the crucible is set at a particular revolution, to thereby uniformize an oxygen concentration profile in silicon monocrystal.
The principle of this method is described in Journal of Crystal Growth, Vol. 96, 1989, pp. 747 and Vol. 98, 1989, pp. 777.
Japanese Unexamined Patent Publication No. 8-231294 has suggested an apparatus for growing silicon monocrystal by Czochralski method. FIG. 1 illustrates the suggested apparatus.
A quartz crucible 104 contains molten silicon 101 therein. The quartz crucible 104 is fixed at a lower surface thereof to a shaft 105 which moves the crucible 104 upwardly and downwardly. A pair of ultra-conductive coil magnets 103 is located around the quartz crucible 104. A central line 102 of magnetic flux generated by the coil magnets 103 horizontally extends. A vertical gap "h" between a surface of the molten silicon 101 and the central line 102 of horizontal flux is kept equal to or smaller than 5 cm while silicon monocrystal is being grown.
Japanese Unexamined Patent Publication No. 8-333191 has suggested an apparatus for growing silicon monocrystal by Czochralski method. FIG. 2 illustrates the suggested apparatus.
In FIG. 2, a crucible 125 contains molten silicon 121 therein, and is connected at a lower surface thereof with a support shaft 122. A pair of ultra-conductive electromagnets 124a and 124b are situated around the crucible 125. The electromagnets 124a and 124b have a common horizontal center line 123. The support shaft 122 is designed to be vertically movable, and raises or lowers the crucible 125 so that the center line 123 of the electromagnets 124a and 124b passes through or below a center of a depth of the molten silicon 121. An equi-intensive curve E has a distribution as illustrated in FIG. 2.
In the above-mentioned apparatuses for growing silicon monocrystal by Czochralski method, a revolution per minute at which a crucible and molten silicon are rotated is defined in accordance with conditions for crystal growth. Hence, if conditions for crystal growth were to be changed, it would be necessary to change the revolution per minute accordingly.
For growing silicon monocrystal having a diameter of 30 cm or greater, it would be necessary to support a crucible containing molten silicon having a great weight, and further necessary to rotate the crucible at a high speed. This causes the apparatus to become large-sized.
For these reasons, it was quite difficult for the conventional apparatuses for growing silicon monocrystal by Czochralski to uniformize oxygen concentration, and prevent fluctuation in quite small concentration of oxygen to thereby draw up crystal having no growth slits therein.
Thus, in accordance with the above-mentioned conventional methods of growing silicon monocrystal by Czochralski, it is necessary to arrange a revolution of a crucible and molten silicon to be greater than a revolution per minute to be determined in accordance with growth conditions, in order to concurrently accomplish both uniform profile of an oxygen concentration in grown silicon monocrystal and prevention of fluctuation in quite small oxygen concentration, that is, prevention of generation of growth slits. As a result, it is quite difficult or almost impossible to grow silicon monocrystal having a diameter of 30 cm or greater.